The present invention relates to soldering flux compositions useful in soldering applications for electronic assemblies such as printed wiring boards.
The manufacture of printed circuit boards (PCBs), also referred to as printed wiring boards (PWBs), is typically divided into two processing categories, fabrication and assembly. Towards the end of the PCB fabrication process, after plating and etching, most of the PCB is covered with a solder mask. The solder mask is used to protect masked areas of the PCB from the attachment of solder. Accordingly, areas of the PCB that will later be soldered, such as the pads and holes, are not covered with the mask.
In a first step of the assembly process, both through-hole and surface mount components, such as integrated circuits, connectors, dual in-line packages, capacitors and resistors, are oriented with the PCB. For example, a component with leads may be mounted on the PCB by placing its leads through holes in the board. Surface mount components can be attached to the board""s bottom side (i.e., the surface to be soldered) using adhesive. The components are then ready to be soldered to the PCB to ensure reliable electrical continuity between the components and PCB circuitry. The component leads and terminations or pads can be soldered via a wave solder process.
The wave solder process includes the steps of fluxing, preheating and soldering. In the fluxing step, a flux is used to prepare the surfaces to be soldered. Such preparation is generally needed because the PCB and the components can become contaminated as a consequence of having been stored in a non-clean-room environment before the soldering process. In addition, oxides may have formed on the leads, terminations and/or pads. In addition to reacting with or removing contaminants and oxides, the flux can perform other functions, such as protecting the surfaces from re-oxidation and reducing the interfacial surface tension between the solder and the substrate to enhance wetting.
Typically, a spray, foam or wave fluxing process is used to apply the flux onto the PCB and component surfaces to be soldered. The fluxing procedure is followed by a preheating step to evaporate the solvent carriers in the flux, such as alcohols or water, and to begin heating the surfaces to be joined. The preheat step is followed by a wave solder process in which the PCB, with components mounted thereon, is passed over a wave of molten solder. The solder wave is pumped through a nozzle; and the wave then contacts and deposits solder on the surface to be joined. The deposited solder then serves to bond and electrically connect the leads and terminations of the components with the contacts on the PCB.
Many of the existing low-solids, no-clean soldering fluxes cause an excessive number of solder balls to be left on the PCB surface. These fluxes also cause solder bridging because their surface tensions are too high. In addition, due to their weak activity levels, the fluxes are unable to thoroughly remove the tarnish and oxides from the surfaces to be joined. Solder balls are unwanted balls of solder occurring randomly or non-randomly on the solder mask and/or between the leads of the components on the board; the solder balls can bridge a gap between two conductors resulting in an electrical short. Solder bridges are connections of unwanted solder that can form a short circuit between two traces or leads that were not designed to be connected. Solder bridges and/or solder balls may cause electrical failure of the board. Excessive solder balls and bridging also require costly solder touch-up operations for their removal. Even tiny solder balls, often referred to as micro-solder balls, because they are only visible with magnification (e.g., 10 times magnification), can result in electrical shorting of very-closely-spaced board lines and pads as well as component leads and terminations. Another problem with existing fluxes is that they can leave visible residues on the surfaces of the PCB, which in addition to being unsightly, can cause false rejects with in-circuit pin testing.
Thus, a need exists for a soldering flux composition that effectively prepares the surfaces to be joined and that reduces the number of solder balls and solder bridges by reducing the surface tension between the PCB surfaces, component leads/terminations and molten solder without increasing the amount of visible residue.
Described herein are fluxes offering reduced-micro-solder-balling, low-residue, low-solids, and no-clean capability. The fluxes include a solvent, an activator in the solvent, a cationic surfactant and a nonionic surfactant. The fluxes are particularly useful for coating a PCB prior to application of solder.
In one embodiment, the solvent is an alcohol, such as isopropyl alcohol. The cationic surfactant can be a quaternary ammonium fluoroalkyl surfactant. The nonionic surfactant can be a nonylphenoxypolyethoxyethanol surfactant. The activator can be a combination of a dicarboxylic acid and a nonionic brominated compound. A printed circuit board coated with a flux described herein includes a substrate on which conductive pathways and conductive contacts, typically formed of metal, are printed and electrically coupled. The flux is coated on the conductive contacts; then solder is applied onto the flux and the board. The solder provides electrical coupling between the contacts and components fixed to the PCB.
Fluxes described herein offer a variety of advantages. The fluxes can effectively remove metal oxides from the PCB surfaces to be soldered so as to promote solder wetting. The fluxes can also substantially lower the interfacial surface tension between the PCB surfaces and a molten solder alloy, thereby promoting drainage of excess solder from the board surface and consequently reducing solder-ball and solder-bridge formation. Moreover, the fluxes can reduce the amount of visible flux residue formed during the wave soldering process.
The soldering flux composition incorporates one or more cationic surfactants, one or more nonionic surfactants and activators dissolved in volatile solvent, such as isopropyl alcohol, ethyl alcohol, de-ionized water or mixtures thereof. Suitable concentration ranges (by weight percent) for these components in the flux composition are as follows: 50-98% solvent, 0.2-10% activator, 0.01-1.0% cationic surfactant, and 0.05-10% nonionic surfactant. In particular embodiments, the concentration range (by weight percent) for each of the above-mentioned components is as follows: 75-98% solvent, 0.2-5.0% activator, 0.05-0.5% cationic surfactant, and 0.10-2.0% nonionic surfactant. One or more high-boiling-point additives can also be incorporated into the flux composition, e.g., at a concentration of 0.2-25% by weight.
A cationic quaternary ammonium fluoroalkyl surfactant, such as FLUORAD FC-135 surfactant (manufactured by 3M Co. of St. Paul, Minn.), SURFLON S-121 surfactant (manufactured by Seimi Chemical Co., Japan), or Neos FTERGENT 300 surfactant (manufactured by Neos, Japan), is used to substantially reduce flux residues as well as the surface tension of the board surface and molten solder alloy. Consequently, the volume of random and non-random solder balls on the board surfaces is also reduced. Ammonium or amine fluoroalkyl surfactants that include an aromatic sulfone functional group (such as Neos FTERGENT 300 surfactant, which is a cationic quaternary ammonium fluoroalkyl compound with an aromatic sulfone functional group) have been found to be particularly effective.
Nonionic surfactants are added to further lower the surface tension of the composition and improve the high-temperature survivability of the flux to further reduce solder balling and bridging. The nonionic surfactant can resist decomposition on a boiling solder pot at a temperature of about 500xc2x0 F. (260xc2x0 C.). Suitable nonionic surfactants include, but are not limited to, nonylphenoxypolyethoxyethanols, Octylphenoxypolyethoxyethanols, alcohol ethoxylates, ethoxylated/propoxylated (EO-PO) copolymers, acetylinic diols, acetylinic diol ethoxylates, perfluoroalkyl ethylene oxides, perfluoroalkyl amine oxides and perfluoroalkyl carboxylates (e.g., S-141, S-145, S-111 and S-113, manufactured by Seimi Chemical Co., Japan).
While either the cationic or nonionic surfactant, by itself, can reduce the surface tension of the molten solder and metal surfaces to be joined, the combination of cationic and nonionic surfactants can produce unexpectedly positive results in terms of avoiding solder balling and solder bridging by dramatically lowering the surface tension of the molten solder and the surface energies of the metal surfaces to be joined, the solder mask on the PWB and the unmasked laminate. In particular, though the use of a nonionic surfactant can offer the desired high-temperature survivability, use of that nonionic surfactant, even at higher concentrations, is likely to still leave a substantial quantity of solder balls on the board surface. In contrast, a combined concentration of nonionic surfactant and cationic surfactant can produce a finished surface that is substantially free of solder balls and solder bridgesxe2x80x94substantially more so than if an equivalent concentration of either nonionic surfactant or cationic surfactant is used in the absence of the other.
The combination of the cationic and nonionic surfactants creates a synergistic effect by lowering both the surface tension of the molten solder and the surface energies of the metal surfaces to be joined, the solder mask on the PWB and the unmasked laminate. Consequently, the amounts of micro-solder balls, bridges and flux residues are reduced to very-low levels.
The flux composition may also comprise various high-boiling-point additives, such as alcohols, glycol ethers and esters to enable the flux to survive the heat of the soldering process in special high-temperature soldering applications. The alcohols may be of various types; for example, aromatic and/or long-chain aliphatic alcohols and rosin alcohols can all be used. Specific examples include benzyl alcohol, tetradecanol and/or hydroabeityl alcohol. Additionally, rosin esters, such as methyl or glycerol or pentaerythritol ester of rosin, esters of modified rosins or resins, such as methyl ester of hydrogenated rosin, and/or other types of esters, such as methyl esters of carboxylic acids, can be included in the flux. Various glycol ether solvents, such as dipropylene glycol monomethyl ether or ethylene/propylene glycol phenyl ether, can also be included in the flux. Rosins, resins and waxes that can be incorporated into the flux include gum rosin, tall oil rosin, wood rosins, chemically-modified resins and rosins (such as hydrogenated rosins), polymerized rosins, fumaric/maleic adducts of rosins, phenolic resins and other natural and synthetic resins, rosins and waxes. These rosins, resins and waxes can be used, alone or mixed together, to enhance electrical reliability of the soldered PWBs.
The flux composition also includes an activator system, which may comprise one or more organic carboxylic acids, such as succinic, adipic, itaconic, malonic, oxalic or glutaric acids. Alternatively, or in addition, the flux can include nonionic, covalent-bonded organic halide activators, particularly bromide activators, such as trans-2,3-dibromo-2-butene-1,4-diol (DBD), styrene dibromide or dibromosuccinic acid. Bromide activators can be mono-, di- or tri-bromo carboxylic acids. The activators chemically remove oxides from the surfaces to be soldered.
The flux can additionally comprise various property-enhancing ingredients in small amounts, which do not materially affect the flux""s basic fluxing properties. Such ingredients include, but are not limited to, corrosion inhibitors, dyes, foaming and/or defoaming agents, biocides and stabilizers. The use of such property-enhancing ingredients is well understood by those skilled in the art of flux formulating.
After the leads or terminations of a component are placed and positioned on a PCB (for example, by feeding the leads through plated or non-plated through-holes in the PCB), the flux can be applied to the leads and contacts by spray, foam, wave or other known methods. An air knife may then be used to remove excess flux, if necessary. After the flux is applied, the board is heated to volatilize the solvent; then the board typically is wave soldered by passing it over a molten solder wave. The wave soldering process deposits solder on the leads and contacts and the solder serves to bond and electrically connect the components to the board.
Flux compositions of the present invention can substantially ensure that the surface tension of the board and the molten solder is lowered during wave soldering so that solder does not form undesirable solder balls and/or bridges on the solder mask and in-between the component leads on the board. Furthermore, cleaning of residual flux from the PCB can be omitted such that the PCB can be used in an electronic application with tiny traces of flux residues remaining on the PCB.
Additional flux components and features, as well as additional methods involving the fluxes and methods described herein, are provided in L. Turbini, xe2x80x9cFluxes and Cleaning,xe2x80x9d Printed Circuits Handbook, 4th Ed., chapter 31 (1996), the teachings of which are incorporated herein by reference in their entirety.
Embodiments of the flux are further illustrated in the following example compositions. It should be understood, however, that although these descriptions particularly describe specific embodiments of the invention, the examples are primarily for purposes of illustration, and the invention, in its broader aspects, is not to be construed as limited thereto.